In the technical field of a nonvolatile memory, much attention is being focused on a ReRAM (resistive RAM). A ReRAM is a resistance variable element generally including a pair of electrodes and a recording film which is selectively switchable between a high-resistance state and a low-resistance state depending on the voltage applied between the paired electrodes. In the ReRAM, the selective switching of the resistance state of the recording film is utilized to record or rewrite information. Examples of such a ReRAM and resistance variable element are disclosed in Patent Documents 1-5 given below.    Patent Document 1: International Publication Pamphlet No. WO 2003/094227    Patent Document 2: Japanese Laid-open Patent Publication No. 2004-273615    Patent Document 3: Japanese Laid-open Patent Publication No. 2004-281913    Patent Document 4: Japanese Laid-open Patent Publication No. 2005-123361    Patent Document 5: Japanese Laid-open Patent Publication No. 2005-203463
ReRAMs are generally divided into a bipolar type and a unipolar type from the viewpoint of the electrical characteristics. In a bipolar type ReRAM, the direction of voltage application between the paired electrodes to shift the recording film from the high-resistance state to the low-resistance state differs from that to shift the recording film from the low-resistance state to the high-resistance state. That is, in a bipolar type ReRAM, voltages of different polarities are utilized in the shifting or switching between the two kinds of resistance states. In a unipolar type ReRAM, on the other hand, the direction of voltage application between the paired electrodes to shift the recording film from the high-resistance state to the low-resistance state is the same as that to shift the recording film from the low-resistance state to the high-resistance state. That is, in a unipolar type ReRAM, voltage of the same polarity is utilized in the switching between the two kinds of resistance states. Generally, the operation speed of a bipolar type ReRAM is higher than that of a unipolar type ReRAM.
As a bipolar type ReRAM, a ReRAM including a recording film made of PrCaMnO3 and a ReRAM including a recording film made of SrZrO3 to which Cr is added have been reported. However, although the fact that these ReRAMs are capable of performing bipolar operation is known, the mechanism of operation has not been figured out. Since the mechanism of operation is unclear, the optimum material, dimension and so on of each part of the ReRAM cannot be determined, so that the optimum designing of the ReRAM is difficult. It is considered that the mechanism of operation of a ReRAM differs largely depending on the kind of the material of the recording film.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a resistance variable element which is capable of performing bipolar operation by a specified mechanism of operation and usable as a memory element.